The present invention relates to a reading system provided in a tracking servo system using a push-pull method in an optical disc player, and more particularly to a system for reading data recorded on an optical disc in the form of a wobbling groove.
At present, write once discs and erasable discs are available for use as writable optical discs having a high recording density. Information is recorded on the disc and reproduced with a laser beam. These discs are different from the CD in the material of the recording surface.
For example, the write once (CD-WO) disc has a tellurium or bismuth recording surface on which the lasers burn pits for recording. In another type of the CD-WO discs, the lasers are focused on a recording surface coated with a selenium antimony (Sb.sub.2 Se.sub.3) thin film, or an oxide tellurium (TeOx) thin film, or a thin film of organic pigment, changing the reflectivity of the light.
The erasable disc uses as the recording surface, an amorphous alloy made of rare earth metals such as gallium, terbium, and others. In a magneto-optical recording method, the recording surface of the disc is initially magnetized to form a magnetic field in a direction perpendicular to the surface. The laser heats a predetermined area of the disc to elevate the temperature above Curie temperature, which is about 150.degree. C., thereby reversing the direction of the magnetic field. To read the recorded information, the laser is irradiated on the recording surface so that polarized wave front slightly rotates as a result of the Kerr effect. Thus only the polarized wave deflected by the rotation is read by a photodetector, thereby enabling the information to be read.
The CD-WO disc has a pregroove which is formed on the recording surface in a spiral and parallel with the track of recorded data. The pregroove has a wobbled inside wall. The wobbling waves are modulated in frequency for representing absolute time, so that the position of the data on the disc may be indicated. FIG. 4 shows a CD-WO disc 1 with a pregroove 2 having a wobbled inside wall. In order to increase the recording density in the radial direction of the disc, pits 3 for representing recorded data are formed in the pregroove 2.
The pitch of the pregroove 2 is about 1.6 .mu.m. The width of the pregroove 2 is 0.45 .mu.m and the depth thereof is 0.1 .mu.m.
The surface of the disc 1 is coated with a thin film of pigment. The laser beam is focused on the surface to convert the beam into thermal energy, so that the nature of the thin film in the groove is changed, thereby forming the pits 3 thereon. Thus, data is recorded in the groove of the disc by laser beam. The information recorded on the disc 1 is reproduced in accordance with the difference of the quantity of reflected light between the pit and an unrecorded portion.
FIG. 5 shows a conventional absolute time reading device provided in a tracking servo system using the push-pull method. The system has a photodetector 4 for detecting the spot of the reflected beams. The photodetector 4 has two detector elements PD1 and PD2 which are defined by a central boundary line 4a in the tangential direction of the disc. The reflected beam forms a shadow by a hatched area in FIG. 5 on each of the detector elements PD1 and PD2.
The push-pull method is one of the methods for deriving a tracking error signal. Referring to FIGS. 6a to 6c, the push-pull method uses change in distribution of energy in a beam spot which is caused by light diffracted and reflected by the pit 3 on the disc 1 when a laser beam is deflected from a track of the disc. When the laser beam is properly centralized on the track, the light is equally diffracted to the right and the left as shown in FIG. 6b. Thus energy is equally distributed. On the other hand, if the tracking is off-center as shown in FIGS. 6a and 6c, the reflected beams are asystemmetrically diffracted. By obtaining the difference between the distributions of energy, the direction in which the beam is deflected from the track can be determined.
If the track on the disc is properly followed, diffracting the beam as shown in FIG. 6b, the shadows formed on the detector elements PD1 and PD2 have the same area so that the difference therebetween is zero. If the beam is deflected to the left of the pit, thereby giving a diffraction shown in FIG. 6a, the shadow on the detector element PD1 is smaller than the shadow on the detector element PD2. To the contrary, if the beam is deflected to the right, so that the beam diffracts as shown in FIG. 6c, the shadow on the element PD2 becomes smaller than the shadow on the element PD1.
Each of the detector elements PD1 and PD2 produces an output representing the area of the shadow. The outputs are fed to a differential amplifier 5 which generates a positive or negative tracking error signal based on the difference between the outputs. The tracking error signal is applied to a tracking servo 6, thereby operating an actuator 7 to move an optical pickup so as to cause the difference to go to zero. The outputs of the detector elements PD1 and PD2 are further applied to a summing amplifier 9 to produce an RF signal.
In order to detect the absolute time on the disc 1, an absolute time decoder 8 is provided for receiving the tracking error signal from the differential amplifier 5. The tracking error includes the wobbling signal dependent on the wobbling of the pregroove. Since the wobbling signal is frequency-modulated, the absolute time can be detected from the tracking error signal.
In the push-pull method, when the axis of the laser beam is not vertical to the recording surface of the disc, or the objective is moved, or the axis of the laser beam is deflected, the spot of the reflected beam is deflected in the direction perpendicular to the center line 4a. Consequently, the distribution of the energy of the spot received by the detector elements PD1 and PD2 fluctuates, thereby causing the tracking error signal to have a DC offset. Therefore, as shown in FIG. 7a and 7b, the level of an RF signal in the output a from a detector element PD1 is different from the level of an RF signal in the output b from the detector element PD2. Hence an RF signal c (a-b) shown in FIG. 7c remains in the output of the differential amplifier 5. Since the output having the RF signal c is applied to the absolute time decoder 8 and the RF signal acts as noise, a desired absolute time signal shown in FIG. 7d is not obtained at the decoder 8.